An electrically variable transmission (EVT) typically has an input member connected to an engine, as well as one or two motor/generators that are each selectively connected to different members of multiple gear sets to enable one or more transmission operating modes. The transmission operating modes may include various EVT modes, such as input, output, and/or compound split modes, fixed speed ratio modes, and electric-only (battery powered) or EV modes. An EVT mode is an operating mode in which the speed ratio between an input and an output member of the transmission is determined by the speed of one or both of the motor/generators.
An EVT may improve vehicle fuel economy in a variety of ways. For instance, the engine may be turned off at idle, during periods of deceleration and braking, and during periods of relatively low speed or light vehicle load operation to thereby eliminate efficiency losses incurred due to engine drag. Captured braking energy via regenerative braking, or energy stored by one of the motor/generators acting in its capacity as a generator during periods when the engine is operating, is utilized during these “engine off” periods to extend the period or duration during which the engine is off, to supplement engine torque or power, to operate the vehicle at a lower engine speed, and/or to supplement accessory power supplies. Transient demand for engine torque or power is supplemented by the motor/generators during “engine on” periods, allowing for downsizing of the engine without reducing apparent vehicle performance. Additionally, the motor/generators are efficient in accessory power generation, and electric power from the battery serves as an available power reserve allowing operation at a relatively low transmission speed ratio.
Electrically-variable operating modes may be classified as input-split, output-split, compound-split, or series modes. Input-split modes have one motor/generator geared such that its speed varies in direct proportion to the transmission output, and have the other motor/generator geared such that its speed is a linear combination of the input and output member speeds. Output-split modes have one motor/generator geared such that its speed varies in direct proportion to the transmission input member, and have the other motor/generator geared such that its speed is a linear combination of the input member and the output member speeds. A compound-split mode has both motor/generators geared such that their speeds are linear combinations of the input and output member speeds, but neither is in direct proportion to either the speed of the input member or the speed of the output member. A series mode has one motor/generator geared such that its speed varies in direct proportion to the speed of the transmission input member, and another motor/generator geared such that its speed varies in direct proportion to the speed of the transmission output member. There is no direct mechanical power transmission path between the input and output members when operating in series mode, and therefore all power must be transmitted electrically.
In each of the four types of electrically-variable operating modes, or EVT modes, the speeds of the motors are linear combinations of the input and output speeds. Thus, these modes have two speed degrees of freedom, abbreviated hereinafter for simplicity as DOF. Mathematically, the speed and torque equations of these class of modes take the form:
      [                                        T            a                                                            T            b                                ]    =                              [                                                                      a                                      1                    ,                    1                                                                                                a                                      1                    ,                    2                                                                                                                        a                                      2                    ,                    1                                                                                                a                                      2                    ,                    2                                                                                ]                ⁡                  [                                                                      T                  i                                                                                                      T                  o                                                              ]                    ⁢                          ⁢              and        ⁢                                  [                                                            N                a                                                                                        N                b                                                    ]              =                  [                                                            b                                  1                  ,                  1                                                                                    b                                  1                  ,                  2                                                                                                        b                                  2                  ,                  1                                                                                    b                                  2                  ,                  2                                                                    ]            ⁡              [                                                            N                i                                                                                        N                o                                                    ]            where a and b are coefficients determined by the transmission gearing. The type of EVT mode can be determined from the structure of the matrix of B coefficients. That is, if b2,1=b1,2=0 or b1,1=b2,2=0, the mode is a series mode. If b1,1=0 or b1,2=0, the mode is an input split mode. If b2,1=0 or b2,2=0, the mode is an output split mode. If each of b1,1, b1,2, b2,1, and b2,2 are nonzero, the mode is a compound split mode.
An EVT may also contain one or more fixed gear modes. In general these modes result from closing one additional clutch than the number required to select an electrically-variable mode. In these modes, the speed of the input and each motor are proportional to the speed of the output. Thus, these modes have only one speed DOF. Mathematically, the speed and torque equations of these class of modes take the form:
      [          T      b        ]    =                              [                                                                      a                                      1                    ,                    1                                                                                                a                                      1                    ,                    2                                                                                                a                                      1                    ,                    3                                                                                ]                ⁡                  [                                                                      T                  a                                                                                                      T                  i                                                                                                      T                  o                                                              ]                    ⁢                          ⁢              and        ⁢                                  [                                                            N                a                                                                                        N                b                                                                                        N                i                                                    ]              =                  [                                                            b                                  1                  ,                  1                                                                                    b                                  1                  ,                  2                                                                                    b                                  1                  ,                  3                                                                    ]            ⁡              [                  N          o                ]            where a and b are coefficients determined by the transmission gearing. If b1,1 is nonzero, motor A can contribute to output torque during operation in the fixed gear mode. If b1,2 is nonzero, motor B can contribute to output torque during operation in the fixed gear mode. If b1,3 is nonzero, the engine can contribute to output torque during operation in the fixed gear mode. If b1,3 is zero, the mode is an electric-only fixed gear mode.
An EVT may also contain one or more modes with three speed DOF. These modes may or may not include reaction torque sources such that the transmission is capable of producing output torque proportional to engine torque or motor torque. If a mode with three speed DOF is capable of producing output torque, the torques of the engine and any motor connected as a reaction to the engine torque will generally be proportional to the output torque. If a motor is not connected as a reaction to the engine torque, its torque can be commanded to control its speed independently of the transmission input and output speed.
In a mode with three speed DOF, it is generally not possible to easily control battery power independently of output torque. This type of mode produces an output torque which is proportional to each of the reacting torque sources in the system. The fraction of total output power provided by each of the three torque sources may be adjusted by varying the speeds of the motors and input. These modes are hereafter referred to as electric torque converter (ETC) modes in recognition of the fact that power flows to or from the ESD as a function of the output torque and the speed of the engine, output, and one of the motors. Mathematically, the speed and torque equations of this class of modes take the form:
      [                                        T            a                                                            T            b                                                            T            i                                ]    =                              [                                                                      a                                      1                    ,                    1                                                                                                a                                      1                    ,                    2                                                                                                a                                      1                    ,                    3                                                                                ]                ⁡                  [                      T            o                    ]                    ⁢                          ⁢              and        ⁢                                  [                  N          b                ]              =                  [                                                            b                                  1                  ,                  1                                                                                    b                                  1                  ,                  2                                                                                    b                                  1                  ,                  3                                                                    ]            ⁡              [                                                            N                a                                                                                        N                i                                                                                        N                o                                                    ]            where a and b are coefficients determined by the transmission gearing.If a1,1 is nonzero, motor A serves as a reaction member and its torque is proportional to output torque when operating in the ETC mode. If a1,1 is zero, motor A is disconnected and its torque is not determined by the output torque. If a1,2 is nonzero, motor B serves as a reaction member and its torque is proportional to output torque when operating in the ETC mode. If a1,2 is zero, motor B is disconnected and its torque is not determined by the output torque. If a1,3 is nonzero, the engine can contribute to output torque during operation in the fixed gear mode. If a1,3 is zero, the input is disconnected and its torque is not determined by the output torque. If all of a1,1, a1,2, and a1,3 are zero, the mode is a neutral mode that is not capable of producing output torque.
Transmission ratios may be changed in an EVT using a synchronous shift or a clutch-to-clutch shift. In the former, EVT modes are used to change direction of or slew the speed of the engine while transitioning between two transmission ratios. Transitions between modes ordinarily go through intermediate fixed gear ratios forming the boundaries between the modes. In the latter, the EVT can shift directly between desired modes using slipping friction clutches. However, control strategies for executing shifts or transitions in an EVT may have a less than optimal shift time and/or shift efficiency.